The purpose of this project is to develop an optimum system to study the mechanism of cell transformation as well as to obtain an assay system for the assessment of human risk from environmental carcinogens. This project may also lead to finding crucial factors involved in cell transformation. The conditions required for maximum enhancement of carcinogen-initiated transformation by tumor promoters have been established using Balb/3T3-A31-1-1 line. The optimum conditions were very similar to those required for the promotion of skin carcinogens. The potency of various promoters in enhancing transformation generally paralleled their potential to promote skin carcinogenesis. These observations, combined with data on the numbers and properties of the receptor for promoters and the early biochemical response to promoters in A31-1-1 cells, indicate that enhancement by promoters of transformation observed with A31-1-1 cells serves as a model system to study the action of promoters. The systems to detect cocarcinogens, "weak carcinogens," and presumptive "pure initiaters" are being developed using A31-1-1 cells. Improvements of the system for DNA transfection transformation assay system and in human diploid cells have been made.